Harmonic line noise correction for electron energy loss spectrometer

Abstract

Electron Energy Loss Spectrometer including a correction circuit for fundamental and third harmonic line noise is described. Various circuits for creating the correction signals are also described. A method of correcting for fundamental and third harmonic line noise is also described.

What is claimed is: 
     
       1. An electron energy loss spectrometer (EELS) operating from an alternating current (AC) power line voltage having an AC power line fundamental frequency comprising:
 a bending magnet for producing a spectrum of electron energy levels; 
 a direct current bending magnet field excitation source; and 
 a power line noise correction circuit configured to produce a first correction signal at the fundamental AC power line frequency and a second correction signal at a harmonic of the fundamental AC power line frequency. 
 
     
     
       2. The EELS of  claim 1 , wherein said power line noise correction circuit includes a sampling circuit for sampling the AC power line voltage. 
     
     
       3. The EELS of  claim 1 , wherein said power line noise correction circuit includes an electromagnetic field sensing sampling circuit for indirectly sensing AC power line noise in the vicinity of the EELS. 
     
     
       4. The EELS of  claim 1 , wherein said second correction signal at said harmonic of the fundamental AC power line frequency is generated independent of the AC power line voltage. 
     
     
       5. The EELS of  claim 1 , wherein said second correction signal at said harmonic of the fundamental AC power line frequency is produced by waveshaping and filtering either the AC power line signal or a signal proportional to a sensed electromagnetic field at the AC power line frequency. 
     
     
       6. The EELS of  claim 1 , wherein each of the first and second correction signals are independently adjustable in phase and amplitude. 
     
     
       7. The EELS of  claim 1 , wherein each of said first and second correction signals are electrically added to said DC bending magnet field excitation source. 
     
     
       8. The EELS of  claim 1 , wherein each of said first and second correction signals are coupled to said bending magnet field excitation source with a transformer. 
     
     
       9. The EELS of  claim 1 , further comprising a drift tube and wherein amplitude and phase-adjusted versions of said first correction signal and said second correction signal are applied to a DC voltage powering said drift tube. 
     
     
       10. The EELS of  claim 1 , wherein said power line noise correction circuit comprises:
 an analog to digital signal converter; 
 a microcontroller; and 
 a digital to analog signal converter, 
 wherein said microcontroller controls said analog to digital signal converter to:
 sample the AC powerline voltage, and 
 store said samples in a buffer and output said samples from said buffer to said digital to analog signal converter, such that there are produced a fundamental frequency output signal and a harmonic of said fundamental output signal based on said samples of the AC powerline voltage. 
 
 
     
     
       11. The EELS of  claim 10 , wherein said harmonic is the third harmonic of the fundamental AC powerline frequency. 
     
     
       12. An alternating current (AC) power line noise correction circuit for producing a correction signal comprising a fundamental AC power line frequency signal and a harmonic of the fundamental AC power line frequency signal, comprising:
 a line voltage sensor configured to produce a line voltage signal; 
 an analog to digital (A/D) signal converter; 
 a microcontroller having a program memory and a read/write buffer memory; and 
 a digital to analog (D/A) signal converter; 
 wherein said microcontroller is configured to
 sample said line voltage signal at a sample rate, 
 store samples of said line voltage signal in said read/write buffer memory, 
 read out said samples of said line voltage signal from said read/write buffer memory to create a first read out at said AC power line frequency and a second read out a harmonic of said AC power line frequency, 
 combine said first and second readouts to produce a combined output value, and 
 output said combined output value to said D/A converter to produce a combined correction signal. 
 
 
     
     
       13. The AC power line noise correction circuit of  claim 12 , wherein said microcontroller is further configured to phase shift one or both of said first and second readouts from said line voltage signal. 
     
     
       14. The AC power line noise correction circuit of  claim 12 , wherein said microcontroller is further configured to provide individual adjustment of amplitude of one or both of said first and second readouts from said line voltage signal. 
     
     
       15. The AC power line noise correction circuit of  claim 12 , wherein said harmonic is the third harmonic of said line voltage frequency. 
     
     
       16. The AC power line noise correction circuit of  claim 12 , wherein said line voltage sensor is a transformer. 
     
     
       17. The AC power line noise correction circuit of  claim 12 , wherein said line voltage sensor is an electromagnetic field sensor. 
     
     
       18. An electron energy loss spectrometer (EELS) subject to noise associated with an AC power source comprising:
 a bending magnet supplied by a bending magnet excitation current; and 
 the alternating current (AC) power line noise correction circuit of  claim 12 , 
 wherein said combined correction signal is applied to said bending magnet excitation current to correct for the noise associated with an AC power source. 
 
     
     
       19. The method of  claim 18 , further comprising:
 Automatically adjusting one or both of said amplitude and phase of one or both of said fundamental and said harmonic correction signals based on feedback from a spectrum detected by the EELS. 
 
     
     
       20. A method of correcting for AC mains noise in an electron energy loss spectrometer (EELS) comprising the steps of:
 sampling an AC power line voltage, 
 adjusting amplitude and phase of the AC power line voltage to create a fundamental frequency correction signal, 
 generating a signal at a harmonic of the AC powerline voltage, 
 adjusting amplitude and phase of said signal at the harmonic of the AC powerline voltage to create a harmonic correction signal and 
 applying said fundamental and said harmonic correction signal to a bending magnet in the EELS).